EFRW and VNA

[Andrew (grayhat)]

Folks, I've had this idea floating in my skull for some time now... but currently I don't have a VNA available, so I'm asking if someone ever tried what follows

When it comes to EFRW antennas we're still following some rather old and established rules about the radiator length and the use of a 9:1 transformer, not that such "rules" are bad, they just work, but the infos they're based on date back to a time when VNAs didn't even exist or were stuff only available in high level labs

Nowadays we have the luxury of cheap and portable VNA units, so... did someone ever try connecting an EFRW wire with one of the "good lengths" directly to a VNA, w/o a 9:1 or whatever and measure the R and J values on different bands and with different EFRW setups (e.g. the ones mostly used in POTA activation or for at home EFRW installation), such a test may offer some better infos about the true impedance of such an antenna and may allow to optimize both, the "good length" and the transformer ratio

Again, do you know if someone ever tried running such an "experiment" ?

[KL7MJ]

I'll give it a go if we get some clear weather this weekend!

[Andrew (grayhat)]

I'll give it a go if we get some clear weather this weekend!

That should be interesting, my suggestion is to start with one of the "recommended lengths" and analize it to obtain R/J at various frequencies (bands), at that point one may either act on the length or (probably better) decide for a different ratio, for example using a 16:1 instead of a 9:1, at any rate I believe it will be interesting AND useful to the whole Ham community !

[KL7MJ]

Here's what I got from 3 different EFHW setups this weekend

First a 35.5/17 with a 9:1 unun:

35_17_91.png (12.91 KiB) Viewed 1924 times

Then a 35.5/17 without an unun:

35_17_no_balun.png (13.51 KiB) Viewed 1924 times

Finally a 29/17 without an unun:

29_17_no_balun.png (13.07 KiB) Viewed 1924 times

Mike, KL7MJ

[Andrew (grayhat)]

Here's what I got from 3 different EFHW setups this weekend

Interesting, but having the R/J (resistive and reactive) values and/or the impedance value would have been more useful, since with that one could be able to find out a better tranformation ratio for the UnUn, just to make an example, playing with NEC, I found that at some radiator lengths, a 16:1 UnUn (800<->50 Ohm) gives a pretty "flat" SWR curve

[KL7MJ]

35.5/17 with Unun

swr_35_17_unun.png (12.42 KiB) Viewed 1026 times

rj_35_17_unun.png (24.33 KiB) Viewed 1026 times

[KL7MJ]

35.5/17 without Unun

swr_35_17.png (13.48 KiB) Viewed 1026 times

rj_35_17.png (21.35 KiB) Viewed 1026 times

[KL7MJ]

29/17 without Unun

swr_29_17.png (13.03 KiB) Viewed 1026 times

rj_29_17.png (22.53 KiB) Viewed 1026 times

[Andrew (grayhat)]

not totally sure, since I'll need to run more verifications, but I believe that it may be worth trying a 16:1 in place of the 9:1, the resulting SWR curve may possibly become more "flat"

[KL7MJ]

I found a video for a 4:1/9:1/16:1 combo. I'll add it to my to-do list.